Metal detectors typically identify the type of a metal object located in a background environment by analyzing the characteristics of an electrical signal attributable to the target. In particular, some metal detectors identify target type by analyzing the phase relationship between a signal transmitted into the ground and a received signal. For example, some metal detectors use phase detectors, sometimes called "synchronous detectors," to measure the received signal at two different phases, usually 90.degree. apart. When the phase detectors are 90.degree. apart, they are referred to as being in "quadrature." The output of each of the phase detectors is a D.C. signal proportional to the magnitude of the received signal at a selected phase angle. The ratio between these D.C. signals is proportional to the phase relationship between transmitted and received signals.
Conventional metal detectors analyze the measured ratio by sequentially comparing it to a series of expected ratios. Such detectors compare the measured ratio with a series of expected ratios one by one until they find the best match. Typical digital and microprocessor based systems, for example, sequentially analyze measured phase data by stepping through a pre-defined series of ranges to find a best fit. Often, this approach is expensive to implement, causes switching noise and signal delays, and can be less accurate.